Semiconductor devices are classified into memory devices and logic devices. The memory devices store data. In general, semiconductor memory devices may be classified into volatile memory devices and nonvolatile memory devices. The volatile memory devices lose data stored therein upon interruption of power supply and include Dynamic Random Access Memories (DRAMs) and Static Random Access Memories (SRAMs). On the other hand, the nonvolatile memory devices do not lose stored data even when power supply is interrupted. Examples of the nonvolatile memory devices include programmable read-only memories (PROMs), Erasable PROMs (EPROMs), Electrically EPROMs (EEPROMs), and flash memory devices.
Recently, next-generation semiconductor memory devices such as Ferroelectric RAMs (FRAMs), Magnetic RAMs (MRAMs) and Phase-change RAMs (PRAMs) are being developed in line with the tendency to achieve high performance and low power consumption of semiconductor memory devices. Materials constituting such next-generation semiconductor memory devices tend to vary in their resistance values according to current or voltage, and maintain their own resistances even when current or voltage supplies are interrupted.